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Old 28th Sep 2018, 6:28 pm   #1
mindvhsfi
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Default Help Identifying Capacitor

Hi,

Trying to repair an old Philips David RB 381 radio, but i'm having some trouble with this component.

Thanks for any help.
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Old 28th Sep 2018, 7:50 pm   #2
Boater Sam
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Default Re: Help Identifying Capacitor

It is what it says on the tin, 2 microfarad 280 v AC rated, made in 1956.
Are you having problems finding a replacement?
What function is it in the set? I see that it is probably on the mains input as a suppressor, so value is not critical, AC voltage is. An X type ( or 2 in parallel to get the value ) will do fine, 250v AC rated
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Old 28th Sep 2018, 9:00 pm   #3
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Default Re: Help Identifying Capacitor

To replace it I'd be looking in the first instance at "Motor-start" capacitors. They're cheap, easily-available, and pretty much bomb-proof.

Ducati - yes, the same Italian company that makes the nice motorcycles - offers a good range of these, available from Farnell at reassuringly-sensible prices.

https://uk.farnell.com/c/passive-com...s?brand=ducati
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Old 28th Sep 2018, 9:52 pm   #4
mindvhsfi
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Default Re: Help Identifying Capacitor

In the schematic it's just at the mains input right after a 180ohm resistance my doubt was because of that polarity symbol at the capacitor witch i found odd as it looks like a electrolytic but it didn't make sense schematic wise.
Will try to replace with a X type suppressor.
But it was nice to know about those "Motor-start" capacitors, always learning.

Thanks for the infos guys.
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Old 30th Sep 2018, 5:03 pm   #5
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Default Re: Help Identifying Capacitor

Have you found the capacitor to be faulty in any way? I can't see what comes behind the letters MK, but this suggests its a foil capactor, not a paper one. Being from 1968 given the date code 28 (so not from 1956 nor 1966), this could make sense. Even if it is a paper capacitor, those in tin cans with rubber seals tend to liver longer than the moulded ones.
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Old 30th Sep 2018, 8:35 pm   #6
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Default Re: Help Identifying Capacitor

Maybe the correct schematic for the radio can be of help to me. I'm sure a 2uF capacitor isn't used as a mains suppressor. Could be used for a series heater string? Could you give the full model number on the label on the back of your set? RB381 usually implies it could be 22RB381/xx but in that model, no such capacitor is used according to the schematic found at http://www.radiodatabase.nl/toestell.../22RB381_1.pdf. Are the first two digits maybe something else instead of 22? Maybe 28 given that you are from Portugal? Where can I find the schematic you're using?

Edit: Is the model number by any chance 19RB381/xx? If so, that's an Italian set for some export market (determined by the xx, 35 for Portugal specifically). I haven't found a schematic yet, but the capacitor is very likely used in the series heater string.

Also, if there is a reason (heaters not working, heater current incorrect or capacitor found to be electrically leaky) to replace it, a mains suppressor is NOT suitable, if you can find one of this value at all. A motor capacitor (preferrably run, but start will probably suffice) is the best option. With the relatively high mains voltage we have nowadays, a 2.2u replacement (standard value) would really be pushing your luck. 2.0u or a bit less would be best.

Last edited by Maarten; 30th Sep 2018 at 8:57 pm.
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Old 30th Sep 2018, 8:52 pm   #7
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Default Re: Help Identifying Capacitor

Hi

My thoughts were the capacitor in post #1 was used as a capacitive dropper rather than mains suppression, 2uF seems too high for suppression purposes, normally 100nF is typical.

Regards
Symon
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Old 30th Sep 2018, 9:15 pm   #8
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Default Re: Help Identifying Capacitor

Yes, according to a schematic I'm looking at it's in series with a 180 ohm resistor and the heater chain....Mains Live>180 ohm>2uF>Thermistor>Lamp>UCH81>UF89>UCL82>Mains Neutral.

The 180 ohm resistor also feeds the HT rectifier.

That's for 220v operation, for 110-127volt operation the 180 ohm and the 2uF are bypassed (shorted out)

Lawrence.

Last edited by ms660; 30th Sep 2018 at 9:36 pm. Reason: Addition for clarification
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Old 30th Sep 2018, 10:37 pm   #9
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Default Re: Help Identifying Capacitor

Found the diagram that appears to be for your set and yes the cap is a dropper along with resistors for the series heater chain.
If the heater chain is open circuit it could be a valve or a resistor or the cap.
If its a valve or a resistor I would be very wary of replacing parts and running on the 220v setting until you are certain that the cap is not leaky or short circuit, you could blow something else.

Running the set on a variac or transformer at the 110v setting would enable you to see what is and is not working without risking blowing the valves.
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Old 30th Sep 2018, 11:55 pm   #10
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Default Re: Help Identifying Capacitor

Found the schematicin the NVHR library http://nvhrbiblio.nl/schema/Philips_19RB381.pdf . If there is a problem with the heater string, it could also be caused by R32, a NTC.

P.S. To answer one of your initial questions: the little symbol with a + and - in it, is the logo for the Philips components division.

Last edited by Maarten; 1st Oct 2018 at 12:07 am.
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Old 1st Oct 2018, 1:27 am   #11
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Default Re: Help Identifying Capacitor

Very interesting power supply arrangements in that set- voltages robbed from the output valve cathode bias chain for both the solid state FM front end and the magic eye filament, as well as that capacitor dropper!
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Old 1st Oct 2018, 8:10 am   #12
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Default Re: Help Identifying Capacitor

I want to contradict something stated earlier. "Motor start" capacitors are sometimes reversible electrolytics, which are short term rated only. Motor start&run capacitors are suitable for continuous use; it is the "run" bit that signifies suitable for continuous use. Due to the phase of the current in the auxiliary/start winding of the motor they are frequently of 400V or 450V AC rating for use on 240V supply.

The reversible electrolytics are really fairly ghastly things, leak electrically, (not physically) something terrible hence they overheat if left with power on for very long. Most motors using them are specified for 10 starts an hours or something similar
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Old 1st Oct 2018, 10:36 am   #13
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Default Re: Help Identifying Capacitor

Re post #14 (above)
I, (and I suspect quite a few others) were unaware of that difference.
I am contemplating converting a small AC/DC set to series capacitor heater feeding to reduce the heat generated inside the cabinet, and will bear the above in mind when procuring a cap for the job. Cheers, tony.
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Old 1st Oct 2018, 1:12 pm   #14
Maarten
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Default Re: Help Identifying Capacitor

Quote:
Originally Posted by trsomian View Post
I want to contradict something stated earlier. "Motor start" capacitors are sometimes reversible electrolytics, which are short term rated only. Motor start&run capacitors are suitable for continuous use; it is the "run" bit that signifies suitable for continuous use. Due to the phase of the current in the auxiliary/start winding of the motor they are frequently of 400V or 450V AC rating for use on 240V supply.
Bipolar electrolytics could possibly be used to obtain large values in a small volume, but it would surprise me greatly since, as you say, tehy don't last long even at 10 starts per hour. I've never encountered them in normal 1kW and less applications, always paper or plastic foil. Maybe a long time ago someone thought it would be a good idea but it never caught on? The usual difference between start and run capacitors is only the continuous use rating.

Since only 100mA is drawn, I don't think it would really matter in this application but a run capacitor, or a 1000V FKP1 or comparable capacitor would be best.

But we'll have to wait until we hear if the capacitor is really the problem here or just a "let's replace all capacitors so the set will surely work again" (no offense, but this happens a lot and is often the cause for additional defects).
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Old 1st Oct 2018, 2:21 pm   #15
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Default Re: Help Identifying Capacitor

A substitute part like this would be perfectly safe in the application and 100% reliable I would expect, you would need to check the geometry:

https://www.ebay.com/itm/2uF-Run-Cap...8AAOSwIC1Zv88Y
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Old 1st Oct 2018, 2:55 pm   #16
mindvhsfi
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Default Re: Help Identifying Capacitor

Thanks for the reply.
The schematic i'm using is the one refered on post #12.
I don't know if the cap in question is bad i just tought that as it seemed to be a electrolytic it would be bether to replace it all together but now that i know it's not perhaps it's ok.
But there are other problems, there are signs of previous repairs or attempted repairs,

R31 seems to have been replaced with other one it measures 28ohm
R32 NTC is all burned up and measures 380Kohm (what type of NTC can be used ?)
C39 (on the schematic it reads 22K, is it a resistor?? ) it has a 30k resistor charred looking
There is also a 1Mohm resistor across the mains in pararell with C45 that is not on the schematic.

The mains entry area looks a mess, rest of the unit seems untouched, also as mentioned on post #11 the set voltage is 220v but the mains now is 240v i gather that even if the set is ok running it at 240v would be a bad thing ?
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Old 1st Oct 2018, 4:00 pm   #17
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Default Re: Help Identifying Capacitor

Check the value of R31 again just to be sure it's 28 ohms and not 280 ohms, if it is 28 ohms do not apply power to the receiver, the value will need to be increased to suit your mains supply voltage.

If the NTC does measure 380kohm I would say it's shot (no good) If it is shot then to give an idea as to what to replace it with it would be good to know the voltage and current rating of the lamp L1, that way we can work out what the hot resistance of the NTC should be when it's passing the designed heater current of 100mA.

C39....do you mean C49? If so the value on the schematic is given in kilo pF, eg: 22k = 22,000pF (= 0.022uf or 22nF) Replace either with an X Class type or......a polypropylene type that has at least a 1,000 volt DC rating.

The 1 meg resistor in parallel with C45....you need to keep that circuit function, the resistor will discharge C45 in the event it remains charged up to mains voltage at switch off (makes the receiver safer to work on when switched off)

220 volts versus 240 volts operation....The value of R31 can be increased to compensate for an increase in mains voltage.

Lawrence.

Last edited by ms660; 1st Oct 2018 at 4:26 pm.
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Old 1st Oct 2018, 4:46 pm   #18
mindvhsfi
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Default Re: Help Identifying Capacitor

Confirmed R21 it's 28ohm

NTC confirmed 380k it's completly charred from overheating by the looks.
The L1 lamp are actually 2 lamps but i can't see any marking's on them and by the looks they aren't in very good shape either, i measured 1ohm across them.

Yeah my mistake it's C49 i was talking about.

Will keep the 1Meg resistor in place.

Any ideia about the value of R31 that would compensate the increase in mains voltage ?
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Old 1st Oct 2018, 9:03 pm   #19
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Default Re: Help Identifying Capacitor

It's hard to be exact with R31 but somewhere around 300 ohms might be ok for 240 volt operation, the test would be to measure the voltage across the valve heaters after it's been fitted.

I can't comment about the bulbs, I've not seen that type before.

380k for the thermistor would indicate that it's not very well, someone might recognize the type from a photo perhaps? I would expect it's resistance at 100mA to be somewhere between 200 and 300 ohms as a first guess.

Lawrence.
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Old 1st Oct 2018, 10:15 pm   #20
mindvhsfi
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Default Re: Help Identifying Capacitor

Regarding R31 does a 5W resistor suffice? and can you tell me in what pins to take the voltage measurement and what voltages to expect?

Do the lamps take any active role in the working of the receiver? Or in other words can it work without them?

The thermistor is definitly not good barely touched it and it cracked in half almost looks like a fuse but the core is solid looks almost like carbon, will post a photo tomorrow.
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